My roomate hasn't arrived yet and I haven't met him yet but a big package of water in plastic bottles has been delivered to the room for him. My metal reusable water bottle is moldy and I didn't get a chance to buy soap to wash it so I took one of my roomate's waters for myself with the intention that I would reuse it a few times and by then I would've gotten a chance to wash my metal water bottle. I feel like I might be stealing and don't want to get in trouble for it but I'm sure my roomate would consent after I meet him. Should I not do this? I saw this searching on Google and it seems like a big deal: https://www.avvo.com/legal-answers/caught-stealing-under--10-of-merchandise-from-walm-1013930.html
From a legal POV you are stealing, and are in the wrong. The correct thing would be to get his consent (phone call?) prior to taking it.
If you have already taken it, you should replace it ASAP. That does not mean you did not steal it, but it reduces the impact of your actions.
From a practical POV you are setting yourself up for issues with your roommate, and possibly get a bad reputation, but it's unlikely the law would get involved over something as trivial as a bottle of water.